Translations:Benutzer:Arian/Klett-Mini-Test/505/en

The belly segment of insects, by contrast, lacks entirely the limb-organisation that in the human being belongs to the metabolic pole. The limbs reach their highest development in the three pairs of legs (in arachnids, four) and the two pairs of wings in the thorax, and extend all the way into the head-organisation — for instance in the mobile, periphery-oriented antennae and the highly differentiated biting and sucking tools. What physiologically constitutes, in the vertebrates and in the highest degree in the human being, the rhythmic system with pulmonary respiration and cardiac function is entirely absent from the thorax of insects. The centre of the dorsally running blood-vessel system lies in the abdomen. Respiration takes place through openings in the skin distributed across the whole body, to which the tubular system of the tracheae connects. The rhythmic system reduces itself entirely to the movement of the wings and the jointed legs. In the thorax region itself it has developed no proper rhythmic organ-systems — no "true" lung, no "true" heart.